Disclosures About Fair Value of Financial Instruments - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Carrying Amount	$ 383,399	$ 371,023	$ 331,306	$ 233,068
Cash and cash equivalents, Estimated Fair Value	383,399	371,023
Investment securities, available for sale, Carrying Amount	418,945	416,685
Investment securities, available for sale, Estimated Fair Value	418,945	416,685
Loans held for sale, Carrying Amount	1,730,002	1,435,459
Loans held for sale, Estimated Fair Value	1,730,501	1,436,460
Loans receivable, net of allowance for loan losses, Carrying Amount	4,735,279	4,281,241
Loans receivable, net of allowance for loan losses, Estimated Fair Value	4,737,189	4,285,537
FHLB, Federal Reserve Bank and other restricted stock, Carrying Amount	63,514	82,002
FHLB and Federal Reserve Bank and other restricted stock, Estimated Fair Value	63,514	82,002
Derivatives assets, Carrying Amount	10,938	7,552
Derivatives assets, Estimated Fair Value	10,938	7,552
Deposits, Carrying Amount	5,785,194	4,532,538
Deposits, Estimated Fair Value	5,799,266	4,540,507
Federal funds purchased, Carrying Amount	50,000	0
Federal funds purchased, Estimated Fair Value	50,000
FHLB advances, Carrying Amount	985,900	1,618,000
FHLB advances, Estimated Fair Value	988,859	1,619,858
Other borrowings, Carrying Amount	88,250	88,250
Other borrowings, Estimated Fair Value	93,222	92,069
Subordinated debt, Carrying Amount	110,000	110,000
Subordinated debt, Estimated Fair Value	111,100	111,925
Derivatives liabilities, Carrying Amount	16,951	9,716
Derivatives liabilities, Estimated Fair Value	16,951	9,716
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	383,399	371,023
Investment securities, available for sale, Carrying Amount	17,863	24,270
Investment securities, available for sale, Estimated Fair Value	17,863	24,270
Loans held for sale, Estimated Fair Value	0	0
Loans receivable, net of allowance for loan losses, Estimated Fair Value	0	0
FHLB and Federal Reserve Bank and other restricted stock, Estimated Fair Value	0	0
Derivatives assets, Carrying Amount	0	0
Derivatives assets, Estimated Fair Value	0	0
Deposits, Estimated Fair Value	3,497,023	2,820,875
Federal funds purchased, Estimated Fair Value	50,000
FHLB advances, Estimated Fair Value	715,900	1,298,000
Other borrowings, Estimated Fair Value	68,285	66,944
Subordinated debt, Estimated Fair Value	0	0
Derivatives liabilities, Carrying Amount	0	0
Derivatives liabilities, Estimated Fair Value	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	0	0
Investment securities, available for sale, Carrying Amount	401,082	392,415
Investment securities, available for sale, Estimated Fair Value	401,082	392,415
Loans held for sale, Estimated Fair Value	1,680,010	1,335,668
Loans receivable, net of allowance for loan losses, Estimated Fair Value	0	0
FHLB and Federal Reserve Bank and other restricted stock, Estimated Fair Value	63,514	82,002
Derivatives assets, Carrying Amount	10,868	7,509
Derivatives assets, Estimated Fair Value	10,868	7,509
Deposits, Estimated Fair Value	2,302,243	1,719,632
Federal funds purchased, Estimated Fair Value	0
FHLB advances, Estimated Fair Value	272,959	321,858
Other borrowings, Estimated Fair Value	24,937	25,125
Subordinated debt, Estimated Fair Value	111,100	111,925
Derivatives liabilities, Carrying Amount	16,951	9,716
Derivatives liabilities, Estimated Fair Value	16,951	9,716
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, Estimated Fair Value	0	0
Investment securities, available for sale, Carrying Amount	0	0
Investment securities, available for sale, Estimated Fair Value	0	0
Loans held for sale, Estimated Fair Value	50,491	100,792
Loans receivable, net of allowance for loan losses, Estimated Fair Value	4,737,189	4,285,537
FHLB and Federal Reserve Bank and other restricted stock, Estimated Fair Value	0	0
Derivatives assets, Carrying Amount	70	43
Derivatives assets, Estimated Fair Value	70	43
Deposits, Estimated Fair Value	0	0
Federal funds purchased, Estimated Fair Value	0
FHLB advances, Estimated Fair Value	0	0
Other borrowings, Estimated Fair Value	0	0
Subordinated debt, Estimated Fair Value	0	0
Derivatives liabilities, Carrying Amount	0	0
Derivatives liabilities, Estimated Fair Value	$ 0	$ 0